FIG. 1, to which reference is now made, illustrates an embodiment of a conventional curtain coating machine as described for example in European Patent 107 818. In the following, the surfaces of the lip body, which will be discussed in more detail later, and the other surfaces of the coating device, are described with reference to the plane of the curtain. The front face of the coating device, that is to say the surface of the lip over which the coating product flows before leaving the coating device will be assumed to be positioned to the left of the observer.
The surfaces of the coating device are assumed to be portions of planes determined by a rotation in a clockwise direction, or in the opposite direction, with respect to the plane of the curtain itself. The positive angles correspond to rotations in a clockwise direction; the negative angles correspond to rotations in an anti-clockwise direction. The positive surfaces indicate surfaces whose angle of rotation is positive; the negative surfaces indicate surfaces whose angle of rotation is negative. As mentioned above, the front face designates in a general sense the surfaces over which the coating product flows before leaving the coating device, whilst the rear face designates the surfaces over which the coating product is deemed not to flow.
According to the embodiment shown in FIG. 1, a conventional curtain coating device comprises a feeding system from which flows a coating composition comprising at least one layer.
The feeding system shown comprises two slots 2, 3 from which two layers flow, superimposed on a slightly inclined flow surface 4. The device also comprises a lip 5 from which the coating composition leaves the device along a negative front surface 6, forming a curtain. The curtain then flows onto a moving support (not shown), which is driven for example by means of a suitable cylinder. The said lip comprises in addition a rear surface 7, which is negative, at least at its lower part, and a chamfer 8 between the bottom ends of the said front and rear surfaces, the chamfer 8 also defining a negative surface. FIG. 2 shows the lip 5 in more detail. In this example the front face of the lip forms a curve of radius R=48 mm; the angle .alpha. defined by the lower part of the rear surface of the lip with respect to the vertical is 15.degree.; the angle .beta. formed by the chamfer with respect to the horizontal is 15.degree.; the angle .gamma. formed by the tangent at the chamfer on the front face of the lip with respect to the vertical is 40.degree., all these angles being given in absolute values. In reality, according to this patent EP 107 818, the rear face of the lip defines a negative surface, as do the front surface 9 and the chamfer 8, the angles .alpha., .beta., .gamma. being negative with respect to the plane of the curtain. The main problem resulting from such a design lies in the fact that it is not suitable for high flow rates of coating composition. The flow rates suggested in this patent vary in fact between 1 and 1.17 cm3/cm/s, which for some applications may prove insufficient. In reality, with higher flow rates, when the negative angle .gamma. defined by the front surface 9 with respect to the vertical is relatively large, that is to say greater than 10.degree. (and all the more so 40.degree., as suggested in the patent EP 107 818), such a design accentuates the defect in the curtain which is commonly referred to as "curtain bendback". Such a phenomenon describes the fact that the curtain, in general, does not fall vertically from the line of the chamfer where it leaves the lip. In reality, even if the lower end of the lip body is vertical, the curtain formed by the coating composition leaves the lip at a certain angle with respect to the vertical. This angle is referred to as the curtain bendback angle. From the lip to the support to which the coating composition is applied, the path of the curtain may be likened to a parabola, as with any object in free fall. This phenomenon is illustrated in FIG. 3. In the following the curtain bendback will be quantified by the distance between the curtain and the vertical plane passing through the end of the lip, measured at 9 cm from the said end. Table I below illustrates the bendback or deviation BB of the curtain obtained for an angle .gamma. of -15.degree. as a function of the flow rate for two different values of viscosity, the angle .beta. formed by the chamfer with respect to the horizontal being 30.degree.. The length of the chamfer is 1.34 mm. The liquids are mixtures of water, gelatin and surfactants at 40.degree. C. The surface tension is 26 mN/m.
______________________________________ FLOW RATE VISCOSITY (P) (cm.sup.3 /cm/s) BB (mm) ______________________________________ 0.04 1 0 0.04 2.2 17 0.04 4.6 34 0.04 6.4 33 0.77 1 0 0.77 2.2 0 0.77 4.6 12 0.77 6.4 19 ______________________________________
For the two viscosity values it is clear that the curtain bendback increases considerably with the flow rate, particularly for low viscosity values. Such a phenomenon is highly detrimental from the point of view of the quality of the curtain and consequently the quality of the coating obtained, and considerably increases the complexity of the coating process. This curtain bendback in fact makes it difficult to accurately predict the point of impact of the curtain on the support to be coated. Consequently, tricky adjustments have to be made in the positioning of the coating device so as to take account of this deviation of the curtain. In addition, this bendback tends to bring the curtain significantly closer to the frame of the coating device. This means that sufficient space has to be provided in order to avoid the curtain hitting the frame of the coating device. For this purpose, it is necessary to extend the slightly inclined flow surface (4; FIG. 1). This is not desirable because of the instability appearing on this surface, this instability being all the greater, the larger the flow surface. Finally, if the edge rods, arranged on each side of the curtain in the plane of the said curtain so as to guide the edges of the curtain, do not have a suitable shape at the level of the lip of the coating device, a large standing wave will appear in the curtain, which will cause a streak in the coating. The greater the curtain bendback, the larger the standing wave will be.
Another problem which the present invention proposes to resolve is related to the wetting of the rear surface of the lip. As shown in FIG. 3, the coating compound climbs up the rear surface of the lip over a distance referenced WL. This wetting of the rear surface of the lip may cause non-uniform coating exhibiting, for example, streaks. For these reasons it is necessary for the wetting height WL to be as small as possible.
U.S. Pat. No. 4,109,611 recommends the use of a chamfer with a relatively large width, that is to say which may be as much as 2.5 mm. Such an approach also generates a large bendback in the curtain when the latter leaves the lip. In fact, since the width of the chamfer is large compared with the thickness of the liquid layer, the result is that the path of the liquid will be considerably deviated. This deviation will be accentuated by the fact that the liquid leaves the end of the lip with a horizontal velocity component in the direction of the natural deviation of the liquid. The greater the forces of inertia, the greater this effect.
Thus one of the objects of the present invention is to provide a curtain coating device in which the curtain bendback is reduced significantly compared with the devices of the prior art.
Another object of the present invention is to provide a coating device substantially reducing the wetting of the rear surface of the lip.
Other objects of the present invention will become apparent during the following detailed description.